Photoengraving involves the manufacture of photoengraved printing articles. The manufacture of such articles involves coating a metal substrate, e.g., plate, sheet or cylinder with a light-sensitive material. The light-sensitive material is then transformed in selected areas by the action of light with or without the assistance of heat into an etch resistant pattern. This pattern is known in the art as a photo resist pattern. This photo resist pattern comprises portions which can be selectively removed from the metal by dissolving those portions to produce a metal substrate having a photo resist pattern through which bare metal is exposed.
The bare metal pattern is then etched, i.e, metal is removed therefrom, to a sufficient depth to permit a printing process to take place or for the preparation of mats. Typical etching depths range from about 0.004 inch, e.g., for half tones, to about 0.040 inch for letterpress work and up to about 0.150 inch for flexographic masters.
Typically, a plate having both the image (resist protected) and non-image (bare metal) areas is placed in a commercial etcher for etching. Various types of etchers are used and these are generally of such character that droplets of an etching liquid are directed against the surface of the plate to be etched. The etching liquid removes a portion of the bare metal and an etched pattern on the substrate results.
A disadvantage of prior art photoengraving articles formed of conventional photoengraving alloys relates to the presence of zinc in such alloys. When such alloys are etched with an etching bath which contains an etchant oil composition, zinc is released into the etching oil effluent. The presence of zinc in the effluent is a cause of environmental concern.
Prior art photoengraving alloys contain zinc as a means as regulating the etching process, e.g., as a means of regulating the etching speed and also as a means for providing a clean, clear etched pattern. Particularly with the use of early etchant oil compositions, if the photoengraving article were formed of an alloy which did not contain zinc, the resultant pattern on the etched article was not acceptable. Typically, the etched pattern would include pimples, undercut portions resulting from lateral etching and rough edges. As used herein, lateral etching refers to undesirable etching of metal protected by the photoresist.
Significant advancement has ocurred in the field of etchant oil compositions. Thus, etchant oil compositions which include isostearic acid have become available. These are exemplified in U.S. Pat. Nos. 3,935,118; 4,032,379; 3,992,234; 3,922,229; 3,736,197; 3,730,899; 3,728,180; 3,725,158 and 3,725,159.
As used herein, "etchant oil" refers to compositions such as those exemplified in the above patents, which are used to prepare etching baths. As used herein, "etching baths" refers to the composition applied to a photoengraving article to conduct the etching processes defined more fully herein. Because zinc-free magnesium-based photoengraving alloys functioned unacceptably with etching baths containing early etchant oil compositions, it was expected that they would function unacceptably with recent etchant oils.
U.S. Pat. No. 1,028,216 discloses the melting and casting of magnesium in the presence of small amounts of calcium. The calcium is added to the magnesium or an unidentified alloy thereof. Zinc is not a component of the alloys of the examples. The alloys are not disclosed as being useful in photoengraving processes.
U.S. Pat. No. 1,698,647 contains a similar disclosure. The magnesium alloy contains 0.05-0.3 percent calcium.
British Pat. No. 661,759 refers to a laid-open British patent application which discloses a magnesium-based alloy containing 1-12 percent aluminum, 0.05-1 percent manganese and 0.005-0.7 percent zirconium. Up to 5% zinc may also be present. The alloy is formulated with or without the zinc. The alloys are not disclosed as being useful in photoengraving.
U.S. Pat. Nos. 2,185,452 and 2,185,453 disclose magnesium alloys containing about 1-12 percent aluminum, about 0.3-5 percent zinc, about 0.05-0.3 percent manganese and about a 0.03-0.3 percent calcium, the balance being magnesium.
U.S. Pat. No. 3,475,140 discloses a composite printing plate formed of a magnesium alloyed layer bonded to an aluminum layer. The magnesium alloys are standard alloys known as PE alloys, which do not contain calcium.
Canadian Pat. No. 594,432; British Pat. No. 661,759 and Letterpress Platemaking, F. G. Wallis (publication date unknown) disclose magnesium-based photoengraving alloy compositions containing both calcium and zinc.